<html class="reftest-wait">
<title>Layout changes on percentage-based scroll timeline</title>
<link rel="help" href="https://drafts.csswg.org/scroll-animations/">
<meta name="assert" content="Scroll timeline should properly handle
layout changes on percentage-based scroll offset">
<link rel="match" href="animation-ref.html">

<script src="/web-animations/testcommon.js"></script>
<script src="/common/reftest-wait.js"></script>

<style>
  #box {
    width: 100px;
    height: 100px;
    background-color: green;
  }

  #covered {
    width: 100px;
    height: 100px;
    background-color: red;
  }

  #scroller {
    overflow: auto;
    height: 100px;
    width: 100px;
    will-change: transform;
  }

  #contents {
    height: 500px;
    width: 100%;
  }

  #spacer {
    height: 80px;
  }

  .invisible {
    display: none;
  }
</style>

<div id="box"></div>
<div id="covered"></div>
<div id="scroller">
  <div id="contents"></div>
  <div id="spacer" class="invisible"></div>
</div>

<script>
  const box = document.getElementById('box');
  const effect = new KeyframeEffect(box,
    [
      { transform: 'translateY(0)', opacity: 1 },
      { transform: 'translateY(200px)', opacity: 0 }
    ], {
      duration: 1000,
    }
  );

  const scroller = document.getElementById('scroller');
  const timeline = new ScrollTimeline({
    scrollSource: scroller,
    timeRange: 1000,
    scrollOffsets: [CSS.percent(20), CSS.percent(80)]
  });
  const animation = new Animation(effect, timeline);
  animation.play();

  // Moves the scroller to the end point (240px).
  const maxScroll = scroller.scrollHeight - scroller.clientHeight;
  scroller.scrollTop = maxScroll * 0.6;

  // Makes sure that the animation runs on compositor with current scroll offset
  waitForAnimationFrames(2).then(_ => {
    // Adds 80px to scroll height which pushes scroll progress back to 50%.
    const spacer = document.getElementById('spacer');
    spacer.classList.remove('invisible');
    // Makes sure that the change is propagated to the compositor.
    waitForAnimationFrames(2).then(_ => {
      takeScreenshot();
    });
  });
</script>